The present invention relates to a motor drive device and an electric compressor and, more particularly to a motor drive device wherein a permanent magnet is provided in a rotor and an electric compressor having the motor drive device.
A motor in which a permanent magnet is provided in a rotor of the motor has been widely used in, for example, an air conditioner or an electric vehicle. In an inverter device that drives a motor, the so-called vector control is employed in which the current is divided into a magnetic flux component current (or a d-axis current) and a torque component current (or a q-axis current) that is orthogonal to the magnetic flux component current and the two component currents are controlled individually. In the vector control, a coordinate system of three phases u, v and w is converted into a coordinate system of d- and q-axes. The angular position of the rotor needs to be known for the coordinate conversion. Generally, the angular position of the rotor is detectable by an angular position sensor, such as a resolver and a rotary encoder.
For the reduction of the size and cost of the motor, it is preferable that a motor should be controlled without the use of such angular position sensor. Therefore, vector sensorless control has been used in which a motor is controlled by the induced voltage that is calculated based on a d-axis current and a q-axis current.
Japanese Unexamined Patent Application Publication No. 2004-72906 discloses a control scheme performed at a start of a permanent magnet motor in which the vector sensorless control is executed.
In the technique disclosed in the above-identified Publication, a current is flowed through coils of a stator to attract a rotor of the motor when the angular position of the rotor before the motor is started is unknown (angular position determination phase). By so doing, the direction of the magnetic flux formed by the permanent magnet and the direction of the magnetic flux formed by an armature current almost coincide with each other.
Subsequently, the command value for rotational speed is incremented form zero (forcible rotation phase). As a result, the direction of the magnetic flux formed by the armature current is rotated so that a difference in phase is created between the direction of the magnetic flux formed by the armature current and the direction of the magnetic flux formed by the permanent magnet. This difference in phase generates a rotational torque and the rotor starts to rotate accordingly.
When the rotational speed of the rotor has reached a predetermined value, a processing for current switching between a current Id and a current Iq is executed (current switching phase). When the current switching processing ends, the rotation of the rotor is controlled using an estimated angular velocity value (steady operation phase).
In the above technique, however, a large amount of power is consumed in the angular position determination phase, and there exists an initial angular position of the rotor which hinders the determination of the angular position thereof. Examples of such initial angular position include the angular position of the rotor that is located 180 degrees shifted or rotated from the motor current at the initial angular position determination phase. Furthermore, the current switching processing while the rotor is rotating is difficult to execute and, therefore, a high operational load is imposed on the controller.
The present invention, therefore, is directed to providing a motor drive device which consumes less power at a start of a motor and imposes less operational load, and to providing an electric motor.